How Often the Goldmann Applanation Tonometer Should be Checked for Calibration Error?

Purpose: To evaluate the frequency of Goldmann applanation tonometer (GAT) calibration error (CE) check. Methods: One observer at each of the 3 tertiary care ophthalmic centers was involved. The tonometers were checked at baseline for CE. A tonometer was defined as faulty when CE exceeded 2 mm Hg at any testing level. Faulty GATs were repaired in-house. Subsequent CE check was done once per month for 6 months. GATs were divided into 3 groups: group 1 (G1)≤1 year, group 2 (G2)>1 to 10 years, and group 3 (G3)>10 years of usage. Results: In total, 76 slit-lamp mounted GATs were included. The number of GATs in groups 1 to 3 was 19, 36, and 21, respectively. Seven (9.2%) tonometers were faulty at the baseline. None in G1, 5 in G2, and 16 in G3 demonstrated unacceptable CE over the study course (P<0.01). The survival function of G1 tonometers was 1.0 throughout, whereas that (95% confidence interval) of the G2 and G3 tonometers were 0.97 (0.81-0.99) and 0.76 (0.51-0.89) at 1 month, and 0.86 (0.69-0.93) and 0.23 (0.08-0.43) at 6 months, respectively. The probability of CE development increased with increasing age of the tonometer. The frequency of use of the tonometer was not associated with the development of CE (odds ratio=1.00). Conclusions: CE occurs more frequently in older tonometers. Although newer GATs (<1 y old) can be checked twice yearly, GATs older than a year should be checked at least monthly.